High efficiency and high linearity adaptive power amplifier for signals with high papr

ABSTRACT

One embodiment of the present invention provides a system for controlling operations of a power amplifier in a wireless transmitter. During operation, the system receives a baseband signal to be transmitted, and dynamically switches an operation mode of the power amplifier between a high power back-off mode having a first power back-off factor and a normal mode having a second power back-off factor based on a level of the baseband signal.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/619,988, entitled “HIGH EFFICIENCY AND HIGH LINEARITY ADAPTIVE POWERAMPLIFIER FOR SIGNALS WITH HIGH PAPR,” by inventors Hans Wang, Tao Li,Binglei Zhang, and Shih Hsiung Mo, filed 14 Sep. 2012.

BACKGROUND

1. Field

The present disclosure relates generally to a power amplifier used inorthogonal frequency-division multiplexing (OFDM) transmitters. Morespecifically, the present disclosure relates to an adaptive poweramplifier that is capable of achieving high efficiency when amplifyingOFDM signals.

2. Related Art

Orthogonal frequency-division multiplexing (OFDM) technology has becomemore and more popular in recent years because of its many advantages,including frequency efficiency and robustness againstfrequency-selective channel fading in the tough wireless environment.During the past decade, OFDM has become the basis of many standards,such as WiFi, Worldwide Interoperability for Microwave Access (WiMAX),Digital Video Broadcasting (DVB), Long Term Evolution (LTE), TV WhiteSpace (TVWS), etc.

However, OFDM also suffers from some drawbacks. One important problem isthe high peak-to-average power ratio (PAPR) of the transmitted signal.The high peak can result in saturation of the power amplifiers, leadingto non-linear signal distortion. To prevent the non-linear distortion,conventional approaches rely on keeping the power amplifier working inthe linear range by backing-off the output power of the amplifierentirely to accommodate the high peaks. Such approaches can result ineither a low signal-to-noise ratio (SNR) or an oversized and inefficientpower amplifier.

SUMMARY

One embodiment of the present invention provides a system forcontrolling operations of a power amplifier in a wireless transmitter.During operation, the system receives a baseband signal to betransmitted, and dynamically switches an operation mode of the poweramplifier between a high power back-off mode having a first powerback-off factor and a normal mode having a second power back-off factorbased on a level of the baseband signal.

In a variation on this embodiment, the system converts the basebandsignal from a digital domain to an analog domain; modulates theDA-converted baseband signal; and amplifies, by the power amplifier, themodulated signal.

In a variation on this embodiment, while dynamically switching theoperation mode of the power amplifier, the system determines whether thelevel of the baseband signal exceeds a predetermined threshold. If so,the system places the power amplifier in the high power back-off mode;if not, the system places the power amplifier in the normal mode.

In a further variation, placing the power amplifier in the high powerback-off mode involves increasing a bias voltage or a bias current ofthe power amplifier.

In a variation on this embodiment, a difference between the first powerback-off factor and the second power back-off factor is determined by apeak-to-average power ratio (PAPR) of the baseband signal.

In a variation on this embodiment, the baseband signal is received froma baseband digital signal processor (DSP) for the wireless transmitter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a diagram illustrating the architecture of aconventional wireless transmitter.

FIG. 2 presents a diagram illustrating the architecture of a wirelesstransmitter, in accordance with an embodiment of the present invention.

FIG. 3 presents a diagram illustrating the architecture of an exemplarypower amplifier controller, in accordance with an embodiment of thepresent invention.

FIG. 4 presents a flowchart illustrating the process of controllingoperations of a power amplifier, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention provide an OFDM transmitter thatcan adaptively adjust the operating point of the power amplifier basedon the level of the signal. More specifically, when the level of thebaseband signal is high, the bias voltage of the power amplifier isincreased to ensure that the high-level signal is not distorted. Oncethe signal level returns to a normal range, the power amplifier returnsto its normal working mode, thus providing higher power efficiencyoverall. Because the decision is made based on the baseband signal, thetransition time between the two operating modes of the power amplifiercan be sufficiently slow.

Power Amplifier for OFDM Transmitter

In OFDM systems, the high PAPR value is a result of the superposition ofmany independent subcarriers, and is directly proportional to the numberof subcarriers. More specifically, the PAPR of an OFDM system can begiven by: PAPR(dB)=10log(N), where N is the number of subcarriers. Forexample, the OFDM-based standard 802.11 a/g specifies the number of OFDMsubcarriers as 52. When the phases of all 52 subcarriers are lined upduring a symbol period, the PAPR is 17 dB. To accommodate such a highpeak while maintaining linearity, that is to provide gain withoutcompression at every possible peak, the operating point of the poweramplifier needs to back-off 17 dB from the peak-power handling point.Such a large power back-off factor means that the power amplifier has tobe oversized in terms of its average power requirements. In addition, itcan only provide a significantly reduced output power (by an amountgiven by the PAPR) when the signal level is lower than the peak. Notethat, because most of the time the signal level is lower than the peak,and because the DC power consumption is determined by the peak level,the overall efficiency of the power amplifier (PA) is very low. Forexample, the maximum power efficiency of a typical Class B PA can be78.5%. However, if the signal being amplified has a PAPR value of 10 dB,this efficiency drops to 7.85%, which means that an output power levelof 100 mW will consume 1.3 W of DC power. Such a high DC powerconsumption can be a huge problem for battery-powered portable devices(such as laptops or tablets) and mobile devices (such as smartphones).

FIG. 1 presents a diagram illustrating the architecture of aconventional wireless transmitter. In FIG. 1, transmitter 100 includes abaseband digital signal processor (DSP) 102, digital-to-analogconverters (DACs) 104 and 106, a radio frequency integrated circuit(RFIC) chip 108, a power amplifier 110, and an antenna 112. RFIC chip108 includes LPFs 114 and 116, variable gain amplifiers (VGAs) 118 and120, mixers 122 and 124, an adder 126, and a power-amplifier driver 128.

During operation, baseband DSP 102 outputs I and Q channel basebanddigital signals to DACs 104 and 106, respectively, which convert thedigital signals to the analog domain. The converted analog signals arethen filtered and amplified by LPFs 114 and 116 and VGAs 118 and 120,respectively. The amplified I and Q baseband signals are then modulatedby a modulator, which includes mixers 122-124 and adder 126. Note thatother standard components of the modulator, such as the local oscillatorand the phase shifter, are not shown in FIG. 1. The modulated signal isthen sent to power amplifier 110 via power-amplifier driver 128. Afteramplification by PA 110, the modulated signal is transmitted via antenna112.

As previously discussed, when designing power amplifier 110, it is verydifficult to simultaneously meet the linearity and power efficiencyrequirements, especially for OFDM signals having high PAPR. Satisfyingthe linearity requirement often means sacrificing power efficiency, andvice versa. To resolve such a conflict, in embodiments of the presentinvention, the operating point of the transmitter PA is adjusteddynamically according to the level of the baseband signal, thus meetingthe linearity requirement while achieving an overall high efficiency.More specifically, in embodiments of the present invention, the PA isconfigured to work at a high power back-off mode only when the systemdetermines that the baseband signal exceeds a threshold; otherwise, thePA is configured to work at a normal mode, which does not require powerback-off.

FIG. 2 presents a diagram illustrating the architecture of a wirelesstransmitter, in accordance with an embodiment of the present invention.In FIG. 2, transmitter 200 includes a baseband digital signal processor(DSP) 202, digital-to-analog converters (DACs) 204 and 206, a poweramplifier controller 208, an radio frequency integrated circuit (RFIC)chip 210, a power amplifier 212, and an antenna 214. RFIC chip 210includes LPFs 216 and 218, variable gain amplifiers (VGAs) 220 and 222,mixers 224 and 226, an adder 228, and a power-amplifier driver 230.

During operation, baseband DSP 202, DACs 204 and 206, RFIC chip 210, PA212, and antenna 214 perform various functions that are similar to theones in the conventional transmitter shown in FIG. 1, includinggenerating I and Q baseband signals, DA-converting the I/Q signals,filtering, modulating, amplifying, and transmitting the modulated radiosignals. In addition, baseband DSP 202 also interacts with PA controller208, which controls the operation of PA 212 based on the level of thebaseband signals.

More specifically, when PA controller 208 detects that the level of thebaseband signal is high (such as exceeding a threshold value), it willmove the operating point of PA 212 to a point that results in PA 212working in a high power back-off mode. In one embodiment, PA controller208 adjusts the bias voltage of PA 212 to a higher level in order tohave PA 212 working in the high power back-off mode. When the level ofthe baseband signal returns to normal, PA controller 208 will then movethe operating point of PA 212 to its normal operating point. Asdiscussed earlier, PA 212 suffers from lower efficiency while working inthe high power back-off mode, because higher bias voltage means higherDC power consumption. However, because for an OFDM system thepossibility of the signal level being much larger than average isrelatively low, and most of the time the signal level remains closed toor lower than the average level, PA 212 only needs to work in this highpower back-off, thus low efficiency, mode for a small percentage oftime. Therefore, the overall efficiency can still remain high.

For example, in a typical OFDM system, 95% of the time the level of thesignal remains closed to or lower than an average level with PAPR muchlower than 10 dB, whereas only during the remaining 5% of the time doesthe PAPR of the signal exceed 10 dB. Consequently, the PA is placed in anormal working mode 95% of the time and in the high power back-off modewith at least 10 dB power back-off factor 5% of the time. For a typicalClass B PA, this means that the efficiency of the PA remains at the78.5% level 95% of the time and only drops to about 7.85% for 5% of thetime. As a result, the overall efficiency is averaged at around 75%,which is more than eight times the 7.85% efficiency of a conventionalPA.

FIG. 3 presents a diagram illustrating the architecture of an exemplaryPA controller, in accordance with an embodiment of the presentinvention. In FIG. 3, PA controller 300 includes a receiving mechanism302, a peak detector 304, and a control-signal output mechanism 306.

Receiving mechanism 302 is responsible for receiving informationassociated with the level of the baseband signals from the baseband DSP,which is the one responsible for generating the baseband signals. If themodulation scheme is quadrature modulation, the overall signal level isdetermined by both the I channel signal and the Q channel signal. Morespecifically, the amplitude of the overall signal can be given as:A=√{square root over (I²+Q²)}, where A is the amplitude of the overallsignal, I is the amplitude of the I channel signal, and Q is theamplitude of the Q channel signal. Note that the I and Q signals remainin the digital domain in the baseband DSP, hence the overall signallevel is also calculated in the digital domain.

Peak detector 304 detects the existence of signal peaks that may resultin the PA being placed in high power back-off mode. In one embodiment,peak detector 304 determines whether the overall signal level exceeds apredetermined threshold. If so, peak detector 304 instructscontrol-signal output mechanism 306 to output a control signal thatplaces the PA in the high power back-off mode. In one embodiment,control-signal output mechanism 306 outputs a control signal configuredto increase the bias voltage of the PA in response to the detection of asignal peak. In a further embodiment, the bias voltage is increased to apredetermined higher value. While operating in the high power back-offmode, the amplifier has a relatively large power back-off factor, whichmeans the operating point of the amplifier is backed-off from the pointthat can produce the maximum average output power without distortion, orthe 1 dB compression point (P1 dB). The power back-off factor is usuallydetermined based on the PAPR value of the signal to be amplified. Forexample, if the PAPR of the signal is 10 dB, then the power back-offfactor needs to be at least 10 dB. Note that the PAPR of the signal canbe predetermined based on the currently active standard, or can beextracted from the baseband signal.

Peak detector 304 continues to monitor the level of the baseband signaland determines whether the overall signal level has returned to normal.In one embodiment, peak detector 304 determines whether the overallsignal level is less than the predetermined threshold. If so, peakdetector 304 instructs control-signal output mechanism 306 to output acontrol signal that places the PA in the normal mode. In one embodiment,control-signal output mechanism 306 outputs a control signal configuredto decrease the bias voltage of the PA in response to peak detector 304detecting the signal level returning to normal. In a further embodiment,the bias voltage is decreased to a predetermined lower value. Whenoperating in the normal mode, the amplifier has a relatively small powerback-off factor, which can be closed or equal to 0 dB, that is no powerback-off is needed in the normal mode.

Control-signal output mechanism 308 is responsible for outputtingappropriate control signals to adjust the operating point of the PA.Note that different types of control signals may be needed for differenttypes of amplifiers. For example, some amplifiers may require adjustingbias voltage, whereas some amplifiers may require adjusting biascurrent.

FIG. 4 presents a flowchart illustrating the process of controllingoperations of a power amplifier, in accordance with an embodiment of thepresent invention. During operation, the system receives basebandsignals from the baseband DSP (operation 402), and calculates an overallsignal level (operation 404). The signal level can be expressed in termsof signal intensity or amplitude. Subsequently, the system determineswhether the signal level exceeds a predetermined threshold (operation406). If so, the system outputs a control signal to the PA to place thePA in a high power back-off mode (operation 408). Otherwise, the systemoutputs a control signal to the PA to place the PA in a normal mode(operation 410). Note that the control signals can be configured toadjust the bias voltage or current of the PA.

Note that the architectures shown in FIGS. 2 and 3, and the processshown in FIG. 4 are merely exemplary and should not limit the scope ofthis disclosure. For example, in FIG. 2, the transmitter implements aquadrature modulation scheme. In general, any type of modulation schemeis also possible. In addition, in FIG. 2, the PA controller is astandalone unit. In general, the PA controller can either be astandalone unit or a part of the baseband DSP. For example, the PAcontroller can be implemented as a function block in the baseband DSP.Moreover, FIG. 4 shows the amplifier being switched between twooperation modes, the high power back-off mode and the normal mode. Ingeneral, it is also possible to fine-tune the operation point of theamplifier so that the amplifier switches among more than two operationmodes. For example, it is also possible to include a medium powerback-off mode, and when the signal level is in a medium range, thesystem places the amplifier in the medium power back-off mode.

Also note that, in embodiments of the present invention, the systemadaptively switches the operating mode of the PA based on the signallevel of the baseband signal, which is transmitted at a data rate muchlower than the carrier frequency. As a result, the control circuit doesnot need to be a high-speed circuit; therefore, the proposed solution isreadily to be implemented using various existing circuitry technologies.It is also optional to introduce a delay circuit in the PA controller tocompensate for the time delay caused by DA-converting and modulating ofthe baseband signal.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium.

Furthermore, the methods and processes described below can be includedin hardware modules. For example, the hardware modules can include, butare not limited to, application-specific integrated circuit (ASIC)chips, field-programmable gate arrays (FPGAs), and otherprogrammable-logic devices now known or later developed. When thehardware modules are activated, the hardware modules perform the methodsand processes included within the hardware modules.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit this disclosure.Accordingly, many modifications and variations will be apparent topractitioners skilled in the art. The scope of the present invention isdefined by the appended claims.

What is claimed is:
 1. A method for controlling operations of a poweramplifier in a wireless transmitter, comprising: receiving a digitalbaseband signal; in response to determining that a power level of thereceived digital baseband signal transitions from being equal to or lessthan a predetermined threshold to being greater than the predeterminedthreshold, placing the power amplifier in a high power back-off modehaving a first power back-off factor; and in response to determiningthat the power level of the received digital baseband signal transitionsfrom being greater than the predetermined threshold to being equal to orless than the predetermined threshold, placing the power amplifier in anormal mode having a second power back-off factor, wherein the secondpower back-off factor is less than the first power back-off factor. 2.The method of claim 1, further comprising: converting the digitalbaseband signal to an analog baseband signal; modulating the analogbaseband signal; and amplifying, by the power amplifier, the modulatedsignal.
 3. The method of claim 1, wherein placing the power amplifier inthe high power back-off mode involves increasing a bias voltage or abias current of the power amplifier.
 4. The method of claim 1, wherein adifference between the first power back-off factor and the second powerback-off factor is determined by a peak-to-average power ratio (PAPR) ofthe digital baseband signal.
 5. The method of claim 1, wherein thedigital baseband signal is received from a baseband digital signalprocessor (DSP) for the wireless transmitter.
 6. The method of claim 1,wherein the digital baseband signal includes an inphase (I) componentand a quadrature (Q) component, and wherein the power level isdetermined based on both the I component and the Q component.
 7. A poweramplifier controller for controlling operations of a power amplifier ina wireless transmitter, comprising: a receiving mechanism configured toreceive a digital baseband signal; a power-determination mechanismconfigured to determine a power level of the received digital basebandsignal; and a control-signal output mechanism configured to: in responseto the power level transitioning from being equal to or less than apredetermined threshold to being greater than the predeterminedthreshold, output a control signal to the power amplifier to place thepower amplifier in a high power back-off mode having a first powerback-off factor; and in response to the power level transitioning frombeing greater than the predetermined threshold to being equal to or lessthan the predetermined threshold, output a control signal to the poweramplifier to place the power amplifier in a normal mode having a secondpower back-off factor, wherein the second power back-off factor is lessthan the first power back-off factor.
 8. The power amplifier controllerof claim 7, wherein placing the power amplifier in the high powerback-off mode involves increasing a bias voltage or a bias current ofthe power amplifier.
 9. The power amplifier controller of claim 7,wherein a difference between the first power back-off factor and thesecond power back-off factor is determined by a peak-to-average powerratio (PAPR) of the digital baseband signal.
 10. The power amplifiercontroller of claim 7, wherein the receiving mechanism receives thedigital baseband signal from a baseband digital signal processor (DSP)for the wireless transmitter.
 11. The power amplifier controller ofclaim 7, wherein the power amplifier controller is part of a basebanddigital signal processor (DSP) for the wireless transmitter.
 12. Thepower amplifier controller of claim 7, wherein the digital basebandsignal includes an inphase (I) component and a quadrature (Q) component,and wherein the power-determination mechanism is configured to determinethe power level based on both the I component and the Q component.
 13. Awireless transmitter, comprising: a power amplifier; a baseband digitalsignal processor (DSP); a digital-to-analog converter (DAC); amodulator; and a power amplifier controller, wherein the power amplifiercontroller further comprises: a receiving mechanism configured toreceive a digital baseband signal; a power-determination mechanismconfigured to determine a power level of the received digital basebandsignal; and a control-signal output mechanism configured to: in responseto the power level transitioning from being equal to or less than apredetermined threshold to being greater than the predeterminedthreshold, output a control signal to place the power amplifier in ahigh power back-off mode having a first power back-off factor; and inresponse to the power level transitioning from being greater than thepredetermined threshold to being equal to or less than the predeterminedthreshold, output a control signal to place the power amplifier in anormal mode having a second power back-off factor, wherein the secondpower back-off factor is less than the first power back-off factor. 14.The wireless transmitter of claim 13, wherein the DAC is configured toconvert the digital baseband signal to an analog baseband signal,wherein the modulator is configured to modulate the analog basebandsignal, and wherein the power amplifier is configured to amplify themodulated signal.
 15. The wireless transmitter of claim 13, whereinplacing the power amplifier in the high power back-off mode involvesincreasing a bias voltage or a bias current of the power amplifier. 16.The wireless transmitter of claim 13, wherein a difference between thefirst power back-off factor and the second power back-off factor isdetermined by a peak-to-average power ratio (PAPR) of the digitalbaseband signal.
 17. The wireless transmitter of claim 13, wherein themodulator is a quadrature modulator.
 18. The wireless transmitter ofclaim 13, wherein the power amplifier controller is part of the basebandDSP.
 19. The wireless transmitter of claim 13, wherein the digitalbaseband signal includes an inphase (I) component and a quadrature (Q)component, and wherein the power-determination mechanism is configuredto determine the power level based on both the I component and the Qcomponent.